Motor control system



June 27, 1944. R, KUTZLER ETAL MOTOR CONTROL SYSTEM Original Filed Nov. 20, 1939 2 Sheets-Sheet 1 flnomtors v Roberl 1-3"; Ku'tzler 1 June 27, 1944. TZL R r Re. 22,508

' MOTOR CIONTROLVSYSTEI Original Filed Nov. 20. 19:9 2 sheets-sheet 2 l Pa e a 1 Reissued June 27, 1944 UNITED STATES PATENT i OFFICE MOTOR ooN'rRon SYSTEM Robert J. Kutzler and Le Roy A. Grifllth, Minneapolis, Mim1., assignors to Minneapolis-Honeywell Begulator Company, Minneapolis, Minn., a corporation of Delaware 28 Claims.

The present invention relates to a motor control system and more particularly to one of the follow-up type.

A common form of motor control system of the follow-up type employs a control impedance and a fellow-up impedance, the follow-up impedance being adjusted by the motor, and the entire control system functioning to position the motor in accordance with the setting or value of the control impedance. The control impedance is often controlled automatically by some condition responsive device. While such a system is quite satisfactory and provides for a large number of positions of the control motor, the system becomes much less satisfactory in operation when it is necessary to compensate the system in accordance with other conditions. Thus in the temperature control art, it is often necessary to control, the motor driving a temperature regulating device in accordance with several diil'erent conditions such as room temperature, relative humidity, and outdoor temperature. The usual practice in compensating a system, of the type discussed is to introduce additional impedances. Certain of these impedances are introduced for reducing the controlling portionof the range of the main control impedance and others areintroduced for providing the compensating action. The result of the introduction of these various impedances is that the sensitivity of the entire circuit is reduced and that the number of available positions is correspondingly diminished.

An object of the present invention is to provide a motor control system of the follow-up type in which the effect of a control impedance is varied, without appreciable change in the impedance of the control circuit, by the introduction of a, variable voltage in the circuit.

A further object of the present invention is to provide such a system in which the amount of movement of the motor for a givenadjustment oi the control impedance is changed by the introduction of a variable voltage in the circuit.

A further object is to provide such a system in which the position of the motor'for a given adjustment of the control impedance is changed by the introduction of a variable voltage in the circuit. 3

A further object of the invention is to provide such a system in which the control impedance is varied in accordance with the value of one condition and in which the.variable voltage introduced inlthe circuit in varied in accordance with the value of a second condition.

A still further object of the invention is to provide such a system in which the control impedance is varied in accordance withthe value of one, condition 'and in the control circuit of which a variable control voltage is introduced, which voltage is varied in accordance with aplurality of compensating conditions.

A still further object of the invention is to provide such a control system for a plurality of motors in which a single control impedance controls the position of all the motors and in which the motors may be individually controlled through variable voltages associated with the individual motor control circuits.

Other objects of the present-invention will be apparent from a consideration of the accompanying specification, claims and drawings of l which:

Figure l is a schematic view of one form of our improved motor control system used for temperature controlling purposes;

Figure 2 is a schematic view of a modified form r of our motor controlling circuit, and

Figure Bis a schematic view of a still different form of our improved system.

Referring to the drawings for a more detailed understanding of the invention, it will be-noted that in Figure 1, the system is shown for purposes of illustration as controllingthe position of a valve M, which valve may be used for regu-' lating the flow of a cooling medium through a pipe ll leading to suitable cooling equipment not shown. The valve I0 is positioned by motor l3.. This motor is of the reversible induction type comprising two rotors I4 and IS with which are associated field windings l6 and I1, respectively. The two rotors are secured to a common shaft l8 which is connected through a reduction gear train 20 to a shaft 2|. Securedto the shaft 2i is a crank-disc 22 which is connected by means of a link 23 to the stem of valve II). It will be obvious that upon rotation of the shaft 2| in a clockwise direction, the valve stem is moved upwardly to open the valve and upon rotation of the shaft 2| in the opposite direction, the valve is moved toward closed position. Motor i3 rotates in one direction or another depending upon which of the two field windings i6 and I! is energized.

The energization of windings l5 and I1 is con-*- trolled by a relay 25. This relay comprises a pair of magnetic cores 25 and 21 which are connected through a suitable connecting member 28 to a switch blade 29. .Switch blade 29 is adapted to'be moved into engagement with either contact 36 or contact 3|. Associated with the core- 26 are two windings 33 and 34. Associated with the core 21 are two windings 36 and 31. The position assumed .by switch blade 26 is dependent upon the relative effective energization of windings 33 and 34 as compared with that of windings 36 and 31. When windings 33 and 34 are more highly energized than windings 36 and 31 the switch blade 26 is moved to the left. whereas when windings 36 and 31 are more highly energized the switch blade 26 is moved to the right. l

A step-down transformer 46 is employed for supplying low voltage power to the relay and to the control system therefor. This transformer comprises a line voltage primary 4| and a low voltage secondar 42. Line voltage primary 4| is connected to linewires 43 and 44.

The two relay windings 33 and 36 are connected in series with each other across the low voltage secondary 42 as follows: from. the right-hand terminal of secondary 42 through conductors 46, 41, relay coil 36, conductors 46 and 66, relay coil 33,

minal of secondary 42 through conductors 46,16,

and conductors 46, 6| and 62 to the other terminal of the secondary 42. Relay coils 33 and 36'are thus constantly energized, the energization being in phase with that of the voltage of secondary 42.

A resistor 56 is connected in parallel with relay coil 33 by conductors 66, 63 and 46. The purpose of resistor 66 is to reduce the energization of relay coil 33 with respect to relay coil 36 so that if the position of cores 26 and 21 is determined solely by the energization of the relay coils 33 and 36, the contact arm 26 is in engagement with contact 3| which, as will appear from the later description, results in the valve being driven to closed position.

The controlling impedance is generally designated by the reference numeral 64. This impedance may be controlled in any suitable man ner but, for purposes of illustration, is shown in the form of a thermostatic potentiometer comprising, a resistance element 65, a contact arm 66, and a bimetallic element 61 adapted to position the contact arm. The'bimetallic element is so disposed that upon a temperature fall the contact arm 66 is moved to the left whereas on a temperature rise the contact arm is moved to the right. This is indicated on the drawings by the legends C and H, the legend C standing for cold and the legend H for hot. The bimetallic element 61 may be made responsive to the temperature of the space being cooled.

A follow-up impedance is designated by the reference numeral 66. This impedance also.

takes the form of a. potentiometer comprising a resistance 6| and a contact arm 62. The contact arm 62 is secured to the motor shaft 2| through an insulated connection 63.

The relay coils 34 and 31 are connected in series between the contact arms 66 and 62 of the control and follow-up potentiometers as follows: from contact arm 66 through the bimetal 61, conductor 66, relay winding 31, conductor 66. transformer secondary winding 66, conductor 16,

relay coil 34, and conductor 1| to contact arm 62. The resistance 56 of the control potentiometer 64 is connected across the low voltage secondary 42 as follows: from the right-hand terminal of secondary 42 through conductors 46, 41, 16, resistance 56, and conductors 16,. 46, 6! and 62 to the other terminal of secondary 42. The resistance 6| of follow-up potentiometer '66 is also connected across the secondary 42 through the following circuit: from the right-hand terand 16, secondarywinding 66, conductor 6|, resistance 6|, conductor 62, secondary winding 64, and conductors 66, 66, and to the other terminal of secondary 42. Disregarding the secondary windings 66, 66, and 64, it will be noted that the potentiometers 54 and 66 together with the. power supply secondary 42 and the relay coils 34 and 31 constitute a bridge circuit wherein the relay coils 34 and 31' are energized by the unbalance current of the bridge. In other words, disregarding the effect of the transformer secondaries 66, 66 and .64, when thetwo contact arms 62 and 66 are similarly positioned there is no potential between them and hence no current flow through relay coils 34 and 31. Upon the position of one of these contact arms being changed with respect to the other, however, a voltage is impressed across the two contact arms, which voltage is either in phase with the voltage of secondary 42 or 180" displaced therefrom depending upon the relative positions of contact arms 66 and 62. Thus if contact arm 66 is to the right of the position occupied by contact arm 62, the voltage between contact arms 62 and 66 is in phase with the voltage of secondary 42 whereas when contact arm 66 is to the left of the position occupiedby contact arm 62, the voltage between contact arm 62 and contact arm 66 is 186 displaced in phase from the voltage across secondary 42.

It will be noted that the coils 34 and 31 are oppositely connected into the circuit between the contact arms 66 and 62. Thus any increase in the current flow through these two coils results in the flux generated by one of the coils aiding its adjacent relay coil and by the other opposing its adjacent relay coil. In other words, if the case is considered in which contact arm 66 is to the right of the position occupied by the contact arm 62, disregarding the effect of the other secondary windings 66, 66 and 64, the current flow through relay coil 34 will be in phase with that through relay coil 33 while the current flow through relay coil 31 will be opposed in phase to that through relay coil 36.- Consequently, coils 33 and 34 will cooperate while coils 36 and 31 will oppose each other. Under these circumstances, thecollective effect of relay coils 33 and 34 will be greater than relay coils 36 and 31 and the switch'blade 26 will be moved into engagement with contact 36. Similarly, it contact through relay coil 34 will be 166 displaced in 2 position with respect to the current flow through relay coil 33. Under these circumstances, the relay coils 36 and 31 will cooperate and will produce a resultant eflect greater than relay coils 33 and 34 which oppose each other.

, The system so far described does not form part of our invention except in so far as our'invention is particularly desirable with and constitutes an improvement of this type of system. In tracing the connections between resistance 6| and the secondary 42, it will be noted that these connections include two secondaries 66 and 64. These two secondaries are two secondaries of a transformer the core of which is indicated by the reference numeralv 66 and the primary of which is indicated by the reference numeral 6|. One terminal of the primary 6|. is connected by conductor 66 to a resistance 62 of a potentiometer having a sliding contact 88. The sliding contact 1 88 is connected to the other terminal of primary 8|. The resistance 82 is connected to the secnected across the secondary 42 and a variable portion of the voltage of secondary 42 is applied to the transformer 9| depending upon the position of tap 88-wh'ich slides over resistance 92. By varying the voltage of primary 9|, the voltage of secondaries 88 and 84 is similarly varied. The two secondaries 88 and 84 are connected in series so that the voltage drops across the two are in the same direction. In other words, let it be assumed that the secondary voltage of transformer 48 is 20 volts so that 20 volts is normally impressed across the relay coils 33 and 38, across the potentiometer resistance 55, and across the potentiometer "resistance 8|. Any variation in the position of contact arm 82 has the same effect as a similar movement of contact arm 58 in the opposite direction. Let it be assumed now that the contact 83 is adjusted, however, so that volts are produced in each of the secondaries 88 and 84. Under these conditions, the 20 volts drop between the two terminals of the. transformer 42 over the path including resistance 8|,

will include volts that is due to the secondaries 88 and 84. The result is that there is only a 10 volt drop across resistance 8I so that a given movement of contact arm 82 has a less effect than a given movement of contact arm 58.

Because of this it is necessaryfor contact arm 82 to move a substantial distance to rebalance a slight movement of contact arm 58 and accordingly movement of contact arm 58 over only a portion of this range of movement effects a full movement of the motor I8 corresponding to a movement of contact arm 82 over its entire range. It will thus be noted that by the introduction of the transformer secondaries 88 and 84 the relative effects of potentiometers 54 and 88'are varied. 'The purpose of this will be apparent from the subsequent description.

The purpose of introducing secondary windings 88 and 84 to reduce the movement of contact arm 58 necessary to effect the full movement ofthe' valve is that it is desired to compensate the action of contact arm 58, or in other .words, to shift the temperature range over which contact arm 58 is effective to control the valve. Thus assume that the contact arm 58 assumes the Position shown when the bimetallicv element 51 is subjected to a temperature of approximately 70. Let it also be assumed that the total range of movement from one end of the resistor to the other takes place over a range of 10. Let it further be assumed that the position shown is a position a quarter of the distance from the righth'and, then only 2 temperature rise is permitted before the contact arm 58 would leave the end of the resistor and cease to have any controlling effect. If it is desired to maintain 70, the contact arm 58 in moving over a 5 range (2 on either side of the position shown) must actuate the valve between its full closed and full openpositions. In other words, if it is desired to shift the control range of the potentiometer 54, it is necessary at any one time to use only a small portion of the control potentiometer and in order to maintain the temperature within this narrow range it is necessary to render the motor capable of operating the valve through its entire range while the control potentiometer is moving over this narrow portion of its range. a

' The compensating means may tak'eany suitable form, being shown here as a thermostatically controlled compensating potentiometer I88. This potentiometer comprises a center, tapped resistance IM and a contactarm I82, which contact arm is positioned by bimetallic element I83. Bimetallic element may, if desired, be responsive to outdoor temperature. The resistance IN is connected by conductors I85 and I88 to the opposite terminals of transformer secondary 42. The potentiometer I88 controls the energization of a transformer I89 comprising secondary 88,. previously mentioned, and primary winding II8. One terminal of primary winding II 8 is connected by conductor III and bimetallic element I83 to the contact arm I82. The other terminal is connected by conductor H2 to the center tap of resistance IN. The compensating potentiometer I88 thus serves to apply a variable portion of the voltage of secondary 42 to the primary H8 and thus to induce a voltage in secondary 88 which is variable in phase and magnitude in accordance with thetemperature to which bimetallic I83 is subjected. Let it be assumed that the bimetallic element moves the contact arm I82 to the right upon a temperature fall as indicated by the legend adjacent the contact arm. Then as the temperature drops, a, voltage will be impressed on winding'til, which voltage will have a phase relation such that it produces the same unbalancing effect as is caused by a movement of contact arm 58 to the right with respect to contact arm 82. Under these circumstances, anew condition of balance will find contact arm 58 to the left of its previously occupied position. In this manner, the entire control range is shifted to the left on resistance 55. If, on the other hand, the temperature adjacent bimetallic element I83 drops so as to cause a movement of contact arm I82 to the left, the voltage impressed on winding '89 will have a phase relation such that it produces the same unbalancing eifect as is caused by a movement of contact arm 58 to the left with respect to contact arm 82. This will cause a shift of the entire control range to' the right on resistance'55.

The three potentiometers 54, 68 and I 88 thus jointly control the energization of the relay which in turn controls the energization of the motor I3. The energization of motor I3 is further controlled by limit switches II5 and H6. Each of these limit switches comprise a pair of switch blades one of which is relatively long to engage with'an arm In of insulating material carried by the shaft 2|. The arm I I] is adapted to engage the long blade of the respective limit switch and to thus open that limit switch when the movement of shaft 2| approaches the end of the desired range of movement of' the control'de'vice.

Operation Let it be assumed that the various elements are shown in the position occupied in which there is substantially no voltage being applied to the primary winding H8 and transformer I89 so that substantially no compensating effect is being introduced into the control circuit for the relay.

The relay is maintained in a balance position due to the fact that the resistor 58 which shunts the coil 33 introduces an unbalancing action, which unbalancing action is compensated for by the fact that the contact arm 58 is very slightly to the right of the position occupied by contact arm 82. Thus consider the situation at the half cycle during which the left-hand terminal of secondary 42 is positive with respect to the right-hand terminal. The current flow under these circumstances is from left to right through relay coils 33 and 36. Due to the slightly unbalanced condition of contact arms 52 and 55, the current flow through relaycoils 34 and 31 is from contact arm 52 through relay coils 3| and 31 to the contact arm 56. This results in the current flow through relay, coil 34 being the same as through relay coil 33 and that through relay coil 31 being opposite to that through relay coil 35. 'Theresult of this ceurrent flow through relay coils 34 and 31 is to increase the effect of relay coil 33 and decrease that of relay coil 36. This counterbalances the opposite effect of shunting resistance 53 so that the relay 25 is maintained in its neutral position.

Now let it be assumed that there is a rise in temperature to which controlling potentiometer 54 is responsive. This temperature may, for example, be the room temperature. The result of this is that the current flow from contact arm 02 to and through contact arm 53 is increased so asto increase the energization of relay coils 34 and 31 and to hence cause the combined effect of relay coils 33 and 34 to begreater than that of relay coils 30 and 31. This will result in movement of switch blade 29 to the left into engage-' ment with contact 30. A circuit is now estab lished to motor field winding It as follows: from line wire 43 through conductor I20, switch blade 23, contact 30, conductor I 2|, limit switch 6, conductor I22, motor winding I3, and conductors I24 and I25 to the other line wire 44. The result of the energization of field winding I5 is to cause the motor I3 to rotate in a direction such as to cause clockwise rotation of shaft 2|.

This clockwise movement of shaft 2| rotates crank-disc 22 in a clockwise direction to move the-valve I0 towards open position and cause more cooling action. The same clockwise movement of shaft 2| also causes the movement of contact arm 32 to the right. This movement continues until the position of contact arm 62 corresponds to the new position of contact arm 55 considering the effect of resistance 50. -When this position is reached, the relay will again be rebalanced and switch blade 23 will separate from contact 30. If the system'did not become rebalanced before valve l0 reached its full open position, the arm H1 would engage the long blade of limit switch H5 and open the motor circuit just traced.

If, on the other hand, the temperature to which bimetallic element 51 is subjected falls, then the current flow through relay coils 34 and 31 will be decreased or even shifted 180 in phase, depending upon the amount of deflection of contact arm 56. In either event, the effect will be to decrease the combined efl'ect of relay coils 33 and '34 and to increase the combined eflect of relay coils 36 and 31. 'The result is that switch blade 29 .will now move into engagement with contact 3| so as to cause the establishment of the following motor circuit to field winding I1: from line wire 43 through conductor I20, switch blade 23, contact 3|. conductor I21, limit switch H5, conductor I28, field winding I1, and conductors I23 and I25 to the other line wire 44. The energize.-

tion of this field winding will cause the motor to rotate in the opposite direction to cause counterclockwise rotation of shaft 2|. This counterclockwise rotation of shaft 2|, with the accompanying counter-clockwise rotation of crank-disc 22, causes valve I0 to, be moved toward closed position to reduce the flow of cooling fluid. The

counter-clockwise. rotation of shaft 2| also causes movement to the right of contact arm 32. This movement takes place until contact arm 62 has reached a position corresponding to the new position of contact arm 53 considering the effect of resistance 50. When the contact arm 52 reaches this position, the relay will again be rebalanced and the energization of the motor will be terminated. If the system should not become rebalanced by the time the valve reaches closed position, limit switch II5 will be opened to terminate the energlzation of the motor.

The operation which has just beendescribed is that which takes place when the compensating potentiometer I00is having no effect whatsoever upon the system, that is, when the outside temperature to which bimetal I03 responds is at an intermediate value such as 85 so that the contact arm I02 is engaging the center tap of resistance IOI. Let it be assumed that the outside temperature drops so that the contact arm I02 is moved to the right to apply a voltage to primary H0 and hence to secondary 63. As previously indicated, this has the same effect as a movement of contact arm 55 to the right. In other words, the combined effect of relay coils 33 and 3| is increased with respect to relay coils 35 and 31 so as to cause switch blade 23 to move into engagement with contact 30. As previously pointed out", this causes a clockwise rotation of shaft 2 I, which in turn results in an opening of the valve and a movement of contact arm 52 to the right. This in turn causes an increase in the delivery of cooling medium which eventually causes a reduction in room temperature and a movement of contact arm 56 to the left. This in turn unbalances the relay in the. other direction so as to cause the motor. to drive back more nearly to its normal position and the movement of contact arm 52 to more nearly the mid position shown. When the system finally becomes rebalanced, contact arm 56 will beto the left of the position shown. In other words, the control range will have been shifted downwardly so that a slightly lower range of temperature is maintained. If, the outdoor temperature rises, however, so as to cause a movement of contact arm "21150 the left, the voltage introduced by secondary 53 will be displaced 180 in phase from the' voltage introduced by secondary 53 upon a drop in outdoor temperature. The result is that the effect will be opposite to that just described so that the control range will be shifted upwardly. In the type of temperature control system with which our improved motor control system is shown for illustrative purposes in Figure 1, this is quite desirable. In order to avoid shock to the person entering a cooled building, it isquite customary to shift the temperature which is maintained in accordance with outside temperature. The purpose of this is to reduce the difference-in temperature between the interior of the building ,and the outside. The control system of Figure 1 thus provides for increasing or decreasing the inside temperature as the outside temperature increases or decreases. It is to be understood, however, that ,the present invention does not rely upon the in: will Illa! BEST AVAILABLE bod 3.. it will noted that the connection of rells y #661118! and .15! between the two contact arms theconnection of relay coils II and ziniimemeoiesof Figure l with the exmum, maria-nae mature directly connected tween unmet arms, and arenot assomil with secondary of=pa transformer. On

other the connection of resistance I46 to the arm now" includes two sets of Warner instead of one set. The mine! in WM these transformer secondaries are with the control circuit will now the washed- I tllhemcmdstywindinss I and *I I4 constitute nae secondary W of a transformer whose is designated by the reference numeral I, These secondary windings correspmadin um II and 4 of the previously In other words, these secavoltage which reduces the M 'eter I45 with respect to w Ill. 'lheprimary winding III is eumcdll'l M me Ill, Tap 2" slides over a rew ill, 1mm resistance is connected by M d I15, I18, I11, 2", 205, 2, III, II, I M

and!" to'the secondary I". The at time has the full voltage of sec- III w across it and the primary applied thereto upon the position explained in connection with my windings 84 and 84 o! the Figure transformer secondaries I" and Ill w outages in the same direction which muslin redeem: the voltage drop across reflm ll. 'lhismeans that a greater movennufid I" must occur for a given Md contact arm I42. By adjusting the an up accordin ly, the sensitivity is varied.

and I46 proas the These two secondary m are, mediate with primary winding at 'tliue alarm or the primary winding III a, pair of compensating and 2 The compensating 2' comprises a resistance .element 2, new arm 2. and a bimetallic element lllmm bere'spon ve to outdoor temper ting potentiometer m:

element 2 over which aomhetarmfli actuated by a relative M element 2i. Connected nails low voltage secondary I61 isa resistance dellfllt Ill. This, resistance element has a sliden aging the samefto form a .g The resistance elements 2" and 20! are connected with ash other across the portion between the right-hand end imp-talk! um!!! end to a sliding tap 200 of aa variable porsecondary winding 2i. and

\ of the responsive elements H2 and 2Ii sistance I46.

plied to primary winding '2 whenever the rela-' tive values of the conditions to which condition are responsive is changed.-

Any voltage which is impressed upon primary results in a voltage being impressed the connections from supply secondary winding I61 through resistance coil I46 and back, it will be noted that secondary windings I18 and I are connected in opposition so that these windings do not afiect the voltage drop across re- The two secondary windings do, however, have the efiect of shifting the mid point voltage drop rebalancing portion of the control circuit. To

illustrate this. let the operatioribe considered during the half cycle in which the left-hand end of secondary I6! is positive with respect to the right-hand end thereof. Let it be assumed that there is a 20 volt drop across secondary I and that potentiometer 2M is so adjusted that there are five volt'drops across each of the secondary windings I" and I84. Since the v ings I80 and I84 aid each other, the total drop across thesetwo secondary windings willbe 10 volts so that there will be only 10 volts drop across the resistance I46. Thus proceeding from the left-hand terminal of secondary lil, there will be a 5 volt drop across winding I84, 10 volts across. winding I46, and 5 volts across winding I84. The mid point of the voltage drop will still, however, be the mid point of resistor I48. Now let it be assumed that the relative positions of contact arms 2 and 2|! are so changed that 2 volts are impressed upon each of windings Ill and I85, this voltage being of such phase that the upper end of the secondary winding IIlis the sidered. If we now trace of secondary winding I61 around the circuit ineluding resistance winding I46, there would still ,ing I18 is cancelled out W the sliding contact 2 I 9 by conductors 22L 222, ill, 22!, 228, and 221. l The voltage anal-0W 24. and 2 is the deternhadluflnepoliflon of sliding contact 2 I 9 which nuke-anally adiusted. The primary windis. I is Md between contact arms 2 andflismlhws: from contact arm 2| I through W M 2I2, conductor ill It!!! be from the above connecflills that the m potentiometers 2" and 209 are wild to .form a bridge circuit with the 2I3, primary and'oonductor 2I4 to contact arm be only 20 volts drop and there would still be resistance I48 due to the fact that the two volts introduced by the windby' the opposing and equal voltage introduced by winding I86. The

only 10 volts across mid position is, however, changed since there will now be 2 volts across secondary I18 and 5 volts across winding IBII leaving the mid point of'the. voltage drop around the follow-up portion of thesystem at a'point three tenths of the distance from the left hand end of resistance I48.

It will be seen from the above that the circuit including compensating potentiometers 208 and 2 introduces a voltage into the control circuit which has as its effect the shifting of the relation of the control potentiometer I40 and the follow-up potentiometer I45. 'Such a shifting of the relation between position of the control potentiometer arm The resultant eifect will be to shift the control point of potentiometer I40. The effect of the control potentiometers 2|. and 20! can be adjusted by the adjustment of sliding contact 2|! with rebetween the two ends of the secondary windthese two potentiometers. .results in a new valve position for any given winding 23I is connected to the center tap of resistance 235. The resistance 235 is connected by conductors 238 and 221 and conductors 231,

I88, and 239 across the same portion of resistance 2I8 as resistance elements 2I0 and 2I4 are connected. In other words, the resistance element 235 has applied thereto the same voltage as resistances 2I0 and 2I4 of. potentiometers 208 and 209. A variable portion of this voltage is applied to primary winding. 23I and hence to secondary windings 230' and 229.

The secondary windings 229 and 230 are connected so that the voltages introduced by them aid each other. The result is that they have the same eilect with respect to the relative effects of potentiometers 208 and 209 as secondaries I80 and I84 have with respect to the relative efiects of potentiometers I40 and I45. In other words, if the voltages introduced by the secondaries 229 and 230 are in phase with the potential difference across resistance 2, the portion of the total potential difference between conductors 225 and 222 that exist across resistor 2I4 is decreased since part of the potential difference between conductors 225 and 222 occurs in the secondaries 229 and 230. If on the other hand, the voltages introduced by secondaries 229 and 230 oppose the normal potential drop through resistor 2, the potential drop occurring across resistor 2l4 will be increased. Thus the relative effects of potentiometers 208 and 209 upon the energization of transformer primary 201 are varied by movement of the slider 234. When the slider 234 is moved upwardly with respect to resistance 235, the voltages introduced by secondaries 229 and 230 are in phase with the voltage between conductors 225 and 222 and the efiect of potentiometer 209 is reduced with respect to that of potentiometer 208. If on the other hand the slider 234 is moved downwardly, the opposite effect is produced so that potentiometer 209 has a greater effect than potentiometer 208.

Operation of Figure 2 species these circumstances, the relay is balanced and switch blade I53 is in engagement with neither contact I54 or I55. The resistance I80 is of higher value than in the species of Figure 1 so that contact arm I42 need be only a slight distance from mid-position to overcome the unbalancing effect of resistance I80.

Since the two conditions to which control potentiometers 200 and 209 respond are assuming corresponding values, the voltage introduced by secondaries I18 and I88 is negligible. If either p the outside temperature or the humidity changes, however, a voltage is applied. to winding 201, f which voltage varies in'magn'itude in accordance with the direction of unbalance. Thus if the contact arm 2 is moved to the right with respect to the position occupied by contact arm 2| 5 as a result of a rise in outside temperature. a voltage will be applied to primary 201 which will in turn introduce a voltage in secondaries I18 and I88. Under these circumstances, the voltage of secondary I18 will oppose that of secondary I81 while the voltage of winding I88 will aid that of secondary I81. This will result in the system being balanced when contact arm I41 is in a position to the left of that shown- It may be recalled that from the disclosure of Figure 1, movement'oflcontact arm I41 to the left accomplishes a closure of the valve controlling the supply of cooling fluid. Thus the effect of the compensation introduced'into the system is that ofcausing a decrease in the supply of cooling fluid for the same position of the control arm I42. This will result in an increase in the space temperature so as to cause thesystem to eventually be rebalanced with the contact arm I42 in a new position to the right of that shown. In other words, the increase in outside temperature has resulted in an increase in the room temperature. It will be obvious that if the out side temperature decreases, the opposite effect will result so that the position of contact arm I41 will be shifted to the right with respect to contact arm I42. At any time that it is desired to vary the total amount of compensation, the slider 2I9 can be adjusted so as to vary the voltage applied to the control potentiometers 208 and 209 and thus to vary the unbalance voltage.

The relative eflect of the two compensating potentiometers 208 and 209 can be varied as previously explained by adjustment of slider 234 with respect to slider 235. This adjustment changes the amount and phase of the voltage impressed upon secondary windings 229 and 230 and hence changes the voltage drop across resistance 2I4 with respect to the voltage drop across resistance 2| 0.

h It will be seen that with the system of Figure 2, a motor control system is provided whereby the motor is positioned in accordance with one main control potentiometer and two compensating potentiometers. two compensating potentiometers can be readily varied and the combined effect of'the compensating potentiometers with respect to the main control potentiometer can also be varied. In

every case the losses due to the various ad-' justing impedances are external/ to the control circuit so that the total impedance of the control circuit remains substantially the same.

Species of Figure 3 The relative effects of the circuit, only these elements will be tors. These relays are designated by the reference numerals 299, 251; and 252. 1 Each relay, as

in thecase of relay 25 of the preferred species, comprises .a switch blade cooperating with con-s reversible motor inone direchand side and relay coils 256 and 251 on the right-hand side. Relay 261 comprises relay coils 259 and 269 on the left-hand side and coils 269 and 261 on the right-hand side. Similarly,'relay coil 252 is provided with relay coils 263 and 264 'on the left-hand side and relay coils 265 and 266 on the right-hand side.

A step-down transformer 219 supplies power for the control system. This transformer comprises a low voltage secondary 211 and a line voltage primary 212.

l The main control potentiometer is designated by the reference numeral 216. This control potentiometer comprises a resistance 216 and a contact arm 211. The contact arm 2114s shown as one which is manually positioned although it is to be understood that the same could be automatically positioned if desired.

The two ends of resistance 216 are connected to the opposite terminals of secondary 211. For convenience of description, the resistance 216 and secondary 211 may be regarded as connected by two continuous conductors designated by the 7 reference numerals 299 and 291.

In other words, conductors 299 and 291 are continuous with the various side conductors hereinafter described as connected thereto rather than as separating the conductors 299 and 291 into sections. Connected between conductors 289 and 291 in parallel are relay coils 255 and 251 of relay. 259, the relay coils 259 and 261 of relay coil 251 and the relay coils 264 and 266 of relay coil 252. In other words, the coils thus named are each connected across the source of power so as to be energized in phase with the voltage of secondary 211.

Associated with each of the motors controlled and with each of the relays 259, 251 and 252 is a .rebalancing potentiometer. These rebalancing potentiometers are designated by the reference numerals 299, 291 and 293. The potentiometer 299 comprises a contact arm 294 and a resistance 295, the potentiometer 291 a contact arm 299 and a resistance 291, and the potentiometer 293, a contact arm 299, and a resistance 299.

Associated with each of the follow-up potentiometers 299, 291 and; 293 is a transformerfor.

introducing a control voltage. The transformer associated with follow-up potentiometer 299 is designated by the reference numeral 399 and comprises a primary winding 391 and a pair of secondary windings .392 and 393. The secondary windings are connected in series with the resistance 294 and in opposition to each otherbetween the conductors 299 and 281 connected to thesecondary 211. These connections are as follows:

from conductor 299 through conductor 395, secondary winding 392, conductor 396, resistance 294, conductor 391, secondary winding 393. and conductor 399 to conductor 291. The two'secondary windings 392 and 393 play the same func-- 'tion as the secondary windings 119 and 196 in the control system of Figure 2. In other words,

In each relay, the switch blade these two secondary windings dueto the fact that they oppose each other-do not change the total voltage drop across resistance 294. Their only effect is to shift the mid-point of the voltage drop between conductors 299 and 291 through resistance 294. The relay coils 254 and 256 are connected between contacts 294 and 211 as fol lows: from contact arm 294 through conductor 319, relay coil 254, conductor 311, relay coil 256, and conductors 312 and 313 to contact arm 211. Thus the shifting of the mid-point of the voltage drop across conductors 299 and 291 through resistance 294 causes a shifting in the energization 'of the windings 254 and 256 to unbalance the relay and cause a. movement of the motor together with the movement of contact arm 294.

The amount of voltage introduced by secondaries 392 and 393 is controlled by adjusting theyoltage applied to primary 391 through a. potentiometer 316. Potentiometer 315 comprises a center tapped resistance element 316 and a sliding contact 311. One end of the resistance element 316 is connected directly to conductor 299 and the other is connected by conductor 319 I is controlled by a potentiometer 324.

to conductor 291. The resistance 316 is thus connected across the secondary 211 and the voltage applied to primary 391 is .the variable portion of the secondary voltage 211 determined by the position of contact 311.

Associated with the potentiometer 291 is a transformer 329, the energization of which is controlled by a potentiometer 321. Similarly associated with the follow-up potentiometer 293 is a transformer 323, the energization of which It is to be understood that transformers 329 and 323 correspond in function to transformer 399 and similarly that potentiometers 321 and 324 correspond in function to potentiometer 315.

The control potentiometer 215 constitutes the control potentiometer of each ,motor controlling unit. The connections of relay coils 254 and 256 of relay 259 have been traced. It will be recalled that these relay coils were connected the contact arm of the follow-up associated with that relay.

299 of the follow-up potentiometer 293 and the contact arm 211 of control potentiometer 215 as follows: from contact arm 299 through conductor 336, relay coil 263, conductor 331, relay coil 265, and conductors 339, 334, and 313 to contact arm 311.

The energizations of the upper coils of each relay are thus controlled by the relative position of the control potentiometer contact arm 211 and Operation of Figure 3 species The relays are all shown in their balanced position. In view of the fact that a shunting resistance has been omitted for purposes of simplicity, the relays are balanced when no current whatsover flows into the upper relay coils. Thus potentiometer I stood that if contact 2 II is moved in the opposite I referring to the control system of relay 2", the contact arms 294 and 211 are inthe same relative positions so that no potential difference exists between thes contact arms andconsequently therels no current flow through relay coils 294 and 259. Similarly, the positions of the contact arms 299 and 29B of follow-up potentiometers 29l and 299 also correspond to the position of contact arm 211 of the control potentiometer. If itls desired to change the position of all of the motors simultaneeously by an equal amount, the contact arm 21'! of control potentiometer 215 is shifted. If, for example, the contact arm 2" is shifted to the right, each of the relays 259, 2 and 252 will be unbalanced in such a direction as to draw the armature and switch blade to the left in the: same manner as described in connection with the previous species. Each of the motors will be placedinto operation to cause a movement of the device positioned thereby and to also cause a movement of the contact arm of the associated rebalancing potentiometer to the right. Upon the contact arm of each rebalancing potentiometer assuming a position corresponding to the new position of the contact arm on the control potentiometer 215, the relay associated with that follow-up potentiometer will be balanced and will assume the miidposition. Regardless of the relative speed of the individual motors, the motors will all eventually stop at the position corresponding to the position of the control potentiometer 211. If the control arm 21'! is moved to the left, the motors will all move in the opposite direction by an amount corresponding to the movementof 'the contact arm 2'".

It will thus be'noted from the above that the control potentiometer 215 constitutes a means for moving a plurality of remotely located motors by an equal amount. that in such a multiple motor control system it is desirable to at times control certain motors individually; The potentiometers 9l5, 92l, and 924 provide such a means. As explained previously, these potentiometers control the voltage applied to their associated transformers I90, 920v and 923. The secondaries of these transformers are connected in series with the resistance lement of the associated follow-up potentiometer. Since the secondaries are connected in oppositionto each other, they do not affect the voltage drop across the follow-up potentiometer but merely shift the control point thereof. Thus referring specifically to the control system .involv'ingrelay 25B, the effect of an increase in the voltageapplied to secondaries Hand 39! is to shift'jthe mid point of th potential drop between conductors 280 and, 291 through resistor 294. This mid point is shifted in one direction or the other depending upon the phase of the voltage applied 'to' primary 3! of the transformer 999. If the slider 9|! is moved to the left, the voltage applied to primary 3M will be of such phase that the voltage .introduced in the secondary winding 302 will oppose the voltage of secondary 2'". The voltage introduced in secondary 993 will aid the voltage of the supply transformer. The resultwill be that the voltage drop between conductors 290 and 28l through resistance 295 will be displaced to the right. This will result in the energization of relay 250 being unbalanced in such a direction as to cause energization of the motor in a direction-to drive contact arm 294 to the right; The motorcontrolled by relay 2 is thus controlled bypotentiometer 3|! independently'of potentiometer 215. It will be under- It often happens, however,

direction, the voltage introduced in secondaries 992 and 392 will be in the opposite'direction so that the opposite action will take place. In other words, the electrical center will be shifted to the left and the motor will be driven in such a direction as to shift contact arm 294 to the left.

' Inasmuch as the control systems for relays 2M and 252 are identical to that for relay 250, it will be apparent that potentiometers 92! and 924 are each capable of positioning their associated motor independently of the main control potentiometer.

It is to be noted that the arrangement of Figure 3 provides an arrangement whereby a series of motors may be controlled in unison by a. single control potentiometer and wherein the motors may be individually controlledby individual control potentiometerswlthout the introduction of any appreciable impedance into the control cir- I cuit. The only impedance which is present is the small impedance of the transformer secondaries which impedance is substantially constant regardless of the voltage applied to the primary of the transformer.

Conclusion It will be noted that in each of the spedies of our invention, the action of the motor control system is compensated in accordance with one or more conditions by the introduction of voltages at appropriate points in the system and without appreciably changing the impedance of the system. While the systems have been specifically described in connection with cooling systems, they could be employed in connection with heating systems or in any other case in which a compensated motor control system of the followup type is desired. Where the systems are used in connection with heating, the valve Ill could control the flow of a heat producing fluid such as a fluid fuel or steam and the various controls would be arranged to operate in the manner necessary in heating. In general, while we have shown certain specific embodiments of our, invention, this is for purposes of illustration only and ourinventtion is limited only by the scope of the appended claims.

We claim as our invention:

1. In a motor control system, a motor, control means including a control circuit having a substantially constant impedance, 9. variable control impedance and a variable follow-up impedancepositioned by said motor, said control means bein operative upon a change in value of said control impedance to cause .said motor to run until the position thereof corresponds to the new value of said control impedance, and means for shifting the range of movement of said motor with respect to any given range of values of said control impedance, said last named means com-' prising means independent of said motor for continuously introducing a, variable voltage in said control circuit by induction into said constant impedance.

2. In a motor control system, a motor, a control circuit therefor comprising a variable controlling impedance and a variable follow-up impedance positioned by said motor, and means for changing the ultimate effect of said controlling impedance without materially changing the total impedance of said circuit, said means comprising a transformer having its secondary permanently connected in said control circuit and its primary connected to means independent of said motor for variably energizing said primary.

3. In a motor control system, a motor, a control circuit therefor comprising a variable controlling impedance and a variable follow-up impedance positioned by said motor, and means for changing the eifect of said controlling im-. pedance without materially changing the total impedance of said circuit, said means comprising a transformer having a primary and a pair of secondaries, said primary being connected to means for variably energizing the same and said secondaries being connected in phase opposition in one portion oi' said circuit. 4. In a motor control system, a motor, a control circuit therefor comprising a variable controlling impedance and a variable follow-up impedance positioned by said motor, means for changing the ultimate eflect of said controlling impedance without ,materially changing the total impedance of said circuit, said means comprising a transformer having a primary and a secondary winding, said secondary winding bevoltages on the opposite sides of one of said poing permanently connected in said control cir- I cult, and means including a plurality of other controllers for variably energizing said primary winding in accordancewith the relative positions of said other controllers.

5. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable con'- tact and resistor, a motor controlling means, a

source of power, means connecting the resistors of said potentiometers in paral1e1 with each other to said source of power and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current of said circuit to cause said motor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometer, and means for shifting the range of movement of said motor with respect to any given range of positions of said control potentiometer without materially changing the impedance of said control circuit, said means comprising means independent of said motor for continuously introducing a variable voltage into the connections of said motor controlling means to said contacts 6. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, motor controlling means, a source of power, means connecting the resistors of said potentiometers in parallel with each other to said source of power and said motor controlling means'between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current of said circuit to cause "id motor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometenand means for shifting the range of movement of said motor with respect to any given range of positions of said control potentiometer without materially changing the impedance of, said control circuit, said means comprising means for introducing two opposing simultaneously variable tentiometer resistors in the connections of said resistor to said other potentiometer resistor and said source of power.

'I. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact ,and resistor, motor controlling means, a source of power, means connecting the resistors of said potentiometers in parallel with each other to said source of power and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current oi said circuit to cause saidmotor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position oi! said control potentiometer, and means for shifting the range of movement oi said motor with respect to any given range ot positions of said control. potentiometer without materially changing the impedance of said control circuit;

the connections of said resistor to said other potentiometer resistor and said source of power.

8. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said p0tentiometers comprising av relatively movable contactand resistor, motor controlling means, a source of power, means connecting the resistors of said potentiometers in parallel with each other to said source of power and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsiveto the unbalance current to cause said motor to be variably energized in a manner-dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometer, and means for changing the ultimate extent of movement of said motor for a predetermined movement of said controlpotentiometer without materially changing the impedance of said control circuit, said means comprising means indcpendentmi' said unbalance currentior introducingz-a vaiiable voltage into the connections of oneiof said potentiometer resistors to the other elements of the system. a Y

9.'Inia motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, eachof said potentiometers comprising a relatively movable contact and resistor, motorcontrolling means, a

source of power, meansconnecting theresistors of said potentiometers in parallel with each other to said source of power-and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current to cause said motor to be variably energized in a manner dependent upon said unbalance current so" that the motor assumes a position dependent upon the controlling position of said control potentiometer, and m ns for changing the extent of movement or said motor for apredetermined movement of said control potentiometer without materially changing the impedance of said control circuit, said means comprising means for introducing a pair of variable voltages of the same phase on opposite sides of one 01' said potentiometer resistors into the connections or said resistor to the other elements 01 the system.- a 1 10. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, ,motor controlling means, a source 01' power, means connecting the resistors or said potentiometers in parallel with each other to said source oi power and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current to cause said motor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometer, and means for changing the extent of movement 01' said motor for a predetermined movement of said control potentiometer without materially' changing the impedance of said control circuit, said means comprising a transformer having a primary winding and two secondary windings, said primary winding being connected to means for variably energizing the same, and said secondary windings being connected in the same phase relationship and on opposite sides of one of said potentiometer resistors in the connections or said resistor to the other elements of the system.

11. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each 01' said potentiometers comprising a relatively movable contact and resistor, motor controlling means, a source of power, means connecting the resistors to form a control circuit, said motor controlling means being operative to position said motor in accordance with the relative controlling positions oi! said potentiometers so that said motor will normally, assume a predetermined position dependent upon the controlling position oi said control potentiometer, and means for" shitting the range of movement of said motor with respect to any given range of positions of said control potentiometer without materially changing the impedance of said control circuit, said means comprising, a transformer having a primary winding and two secondary windings, said ondary windings being connected in opposition and on opposite sldes'oi one 01' said potentiometer resistors in the connections oi said resistor to said other potentiometer resistor and saidsource of power, means including a plurality of of said potentiometers in parallel with each other to said source of power and said motor control means to said resistors through said contacts to form a control circuit, said motor controlling means being operative to position said motor in accordance with the relative controlling positions of said potentiometers so that said motor will normally assume a predetermined position dependent upon the controlling position 01 said control potentiometer, and means for shifting the range 01' movement 01' said motor with respect to any given range of positions of said control potentiometer without materially changing the impedance of said control circuit, said means comprising a transformer having a primary winding and two secondary windings, said secondary windings being connected in opposition and on opposite sides of one of said potentiometer resistors in the connections of said resistor to said other potentiometer resistor and said source of power, and means including a plurality of other controllers for variably energizing said primary winding in accordance with the relative positions of said other controllers. I

12. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each 01' said potentiometers'comprising a relatively movable contact and resistor, motor controlling means, a source or power, means connecting the resistors of said potentiometers in parallel with each other to said source of power and said motor control means to said resistors through said contacts other impedances connected in the form of a Wheatstone bridge, and means for applying, the

unbalance voltage oi said bridge to said primary winding.

13. In a motor control system, a plurality of motors, a main variable control impedance, a plurality of follow-up impedances' each positioned by one oi said motors, a motor controlling means associated with each motor and its associated follow-up impedance and with the main control impedance to cause said motor to assume a position dependent upon the position of said controlling impedance so that upon a change in the position of said controlling" impedance, the positions of allot said motors are simultaneously changed correspondingly, and means associated with one of said motors for individually changing the position oi that motor only without changing the impedance oi the entire control circuit, said means comprising means for inducing a variable voltage in the energizing circuit of the associated motor controlling means.

14. In a motor control system, a plurality of motors, a main variable control impedance, ,a plurality of follow-up impedances each positioned by one of said motors, a motor controlling means associated with each motor and its associated follow-up impedance and with the main control impedance to cause said motor to assume a position dependent upon the position of said controlling impedance so that upon a change in the position of said controlling impedance, the positions of all of said motors are simultaneously changed correspondingly, and means associated with one of said motors for individually changing the position 01' that motor only without changing the impedance of the entire control circuit, said means comprising a transformer having its primary connected to .means for variably energizing the same and its secondary in the energizing circuit oi the associated motor controlling means.

15. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, motor controlling means, and a control circuit including means connecting said motor controlv means between said contacts, and connections between each terminal 01' said control potentiometer and a corresponding terminal of said follow-up, potentiometer, said connections including means for changing the potential or each said control potentiometer terminal with respect to its corresponding follow-up potentiometer terminal so as to change the ultimate effect of said control potentiometer on said motor, without substantially changing the impedance of said connections.

16. In a motor control system, a motor, a

control potentiometer, a follow-up potentiom-' potential changing means simultaneously and in an opposite sense, thereby shifting the range of operation of said control potentiometer contact with respect to the range of operation of said follow-up potentiometer contact.

17. In a motor control system, a motor, a control circuit therefor comprising a variable controlling impedance and a variable follow-up impedance positioned by said motor, means for changing the ultimate effect of said controlling impedance without materially changing the total impedance of said circuit, said means comprising a transformer having a primary and a secondary, ,winding, said secondary winding being connected'in said control circuit, means including a plurality of other e trollers for variably energizing said primary wi ding in accordance with the relative positions of said'other controllers, and means for adjusting the relative eflects of said other controllers on the energization of said primary winding. I

18. In a motor control system, a motor, a control circuit including a relay controlling the energization of said motor, a variable control impedance and a variable follow-up impedance po sitioned by said motor, said two impedances jointly controlling the energization of said relay to cause the motor to assume a position dependent upon the value of said variable control impedance, and means for changing the ultimate eifect of said controlling impedance without materially changing the impedance of said circuit,-

said means comprising means independent of said motor and said relay for introducing a variable voltage in said circuit between said followup potentiometer and said relay.

19. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, a motor controlling means, a source of power, means connecting the resistors oi said potentiometers in parallel with each other to said source or power and said motor controlling meansbetween said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current of said circuit to cause said motor to be variably energized in a manner dependent upon ,said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometer, and means for shifting the range of movement of said motor with respect to any given range of positions of said control potentiometer, said means comprising means independent of said motorfor continuously introducing a variable voltage into the connections of said motor controlling'means to said contacts.

20. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, a motor controlling means, a source of power, means connecting the resistors of said potentiometers with said source to form it normally balanced electrical network, an impedance device, means connecting said impedance device and said motor controlling means between said contacts to form a control circuit, said motor controlling means being responsive to the unbalance current of said network flowing in said circuit to cause said motor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependentupon the controlling position of said control potentiometer, and means for changing the flow of current in said circuit to shift the range of movement of said motor with respect to any given range of positions of said control potentiometer, said means comprising a compensatingpotentiometer including a relatively movable" contact and resistor, said resistor having terminals and a tap at a'substantially central point thereon, a source of electrical energy connected to said terminals, means for moving said contact relative to said resistor, and connections between said contact, said tap, and said impedance device;

21. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiometers comprising a relatively movable contact and resistor, a motor controllingmeans, a source of power, means connecting the resistors of said potentiometers with said source to form a normally balanced electrical network, an impedance device, means connecting said impedance device and said motor controlling means'between said contacts to form a control circuit. said motor controlling means being responsive to the unbalance current of said network flowing in said circuit to cause said motor to be variably energized in a manner dependent upon said unbalance current so that the motor assumes a position dependent upon the controlling position of said control potentiometer, and means including a potentiometer and a device responsive to a physical condition for adjusting said potentiometer, said last named means normally shunting at least a portion of said impedance device but responsive to a varia--- tion in the value of said condition from a predetermined value to apply a potential across said impedance device whose value is substantially proportional to the deviation of said condition from said predetermined value.

22. In a motor control system, a motor, power supply means including a first transformer secondary winding, controlling means for saidmotor including a control circuit comprising a variable controlling impedance and said winding, a substantially constant impedance having at least a portion thereof connected" in said circuit, said controlling means being eii'ective to cause said motor to operate in a manner dependent upon the voltage across a portion of said control circuit including both said variable controlling impedance and at least said portion of said constant impedance, a second transformer secondary winding, and means normally effectively shunting said constant impedance but responsive to a condition indicative of a need for a change in the ultimate controlling eii'ect of said variable impedance to apply a potential derived from said second winding across said constant impedance.

23. In a motor control system, a motor, consaid constant impedance, and means for chan ing the ultimate controlling effect of said variable impedance, said means comprising a compensating potentiometer including a relatively movable contact and resistor, said'resistor having terminals and a tap at a substantially central point thereon, a source of electrical energy connected to said terminals, means for moving said contact relative to said resistor, and means including electrical connections for introducing into said constant impedance a potential proportional to that existing between said tap and said contact.

24. In a motor control system, a motor, control means including a control circuit having a variable control potentiometer and a variable followup potentiometer positioned by said motor, said control means being operative upon a variation of said control potentiometer position to cause said motor to run until the position of the follow-up potentiometer corresponds to the new position of said control potentiometer, means having a substantially constant impedance for introducing intosaid control circuit a first variable voltage so as to change the ratio'between a given movement of said control potentiometer and the responsive movement of said follow-up potentiometer without affecting the impedance of. said circuit, and means also having a substantially constant impedance for introducing into said circuit a second variable voltage so as to shift the range of movement of said motor with respect to any given range of positions of said control potentiometer without alfecting the impedance of said circuit.

25. In a motor control system, a motor, a control potentiometer, a follow-up potentiometer positioned by said motor, each of said potentiomeometers comprising a relatively movable 'contact' ters comprising a relatively movable contact and resistor, motor controlling means, and a control circuit including means connecting said motor control means between said contacts, and connections between each terminal of said control potentiometer and a corresponding terminal of said follow-up potentiometer, means having a substantially constant impedance for introducing into said control circuit a first variable voltage so as to change the ratio between a given movement of said control potentiometer and the responsive movement of said follow-up potentiometer without affecting the impedance of said circuit, and means also having a substantially constant impedance for introducing into said circuit a second variable voltage so as to shift the range of movement of said motor with respect to .determined functional relationship to any given range of positions of said control potentiomete'r without affecting the impedance of said circuit, said last-named means comprising a transformer secondary winding in said connect n means.

26. In a motor control system, a motor, a control potentiometer, a follow-sup potentiometer positioned by said motor, each of said potentiand resistor, motor controlling means, and a control circuit including means connecting said motor control means between said contacts, and connections between each terminal of said control potentiometer and a corresponding terminal of said follow-up potentiometer, means having a substantially constant impedance for introducins into said control circuit aflrst variable voltage soas to change the ratio between a given movement of said control potentiometer and the responsive movement of said follow-up potentiometer without affecting the impedance of said circuit, and means also having a substantially constant impedance for introducing into said circuit a second variable voltage so as to shift the range of movement of said motor with respect to any given range of positions of said control potentiometer without affecting the impedance of said circuit, said last-named means comprising a pair of transformer secondary windings con-. nected in phase opposition on opposite sides of one of said potentiometer resistors in said connections.

' 27. In a motor control system, a motor, controlling means for circuit comprising a variable controlling impedance, a substantially constant impedance permanently connected in said circuit, said controlling means being effective to cause said motor to operate in a manner dependent upon the voltage across a portion of said control circuit including both said variable controlling impedance and said constant impedance, means external to said circuit for continuously inducing a potential in said constant impedance to determine the ultimate controlling effect of said variable impedance, and means for varying said potential.

28. In a rebalancing motor control system, a motor, a normally balanced control circuit therefor comprising a substantially constant impedance, a variable controlling impedance, and a follow-up impedance variable in value by operation of said motor, means responsive to unbalance of said circuit for causing operation of said motor so as to rebalance said circuit by variation of said follow-up impedance, the rebalancing value of said follow-up impedance having a prethe value of said controlling impedance, and means external to said circuit and consisting of a further variable impedance variable by a device respon- 1 sive to a physical condition for inducing a potential in said constant impedance to change said functional relationship to an extent dependent upon thevalue of said condition.

ROBERT J. KUTZLER. LE ROY -A. GRIFFITH.

said motor including a control 

